A wealth of experimental data argues persuasively that infectious prion particles are composed largely, if not entirely, of the scrapie isoform of the prior protein (PrPSc). Additional investigations have shown that PrPSc is formed from the cellular prion protein (PrPc) by a posttranslational process, which occurs after PrPC reaches the cell surface where it is anchored by a glycosyl phosphatidyl-inositol (GPI) moiety. No candidate chemical modifications have been identified that might distinguish PrPSc from PrPC, to date. These experimental results argue that it is likely that the conversion from PrPC into PrPSc involves a conformational change in the protein. In the studies proposed here, we plan to refine the purification of PrPc and establish whether the primary structure of PrPC including the posttranslational modifications is identical to that already determined for PrPSc. The secondary, tertiary and quaternary structures of PrPC and PrPSc as well as synthetic PrP peptides will be determined using spectroscopic methods. These studies should help elucidate the conformational changes involved in the various "strains" of scrapie are due to detailed differences in the posttranslational modifications of PrPSc including the arrays of heterogeneous asparagine-linked oligosaccharides and the GPI anchor. We propose to investigate the possibility of generating scrapie infectivity in vitro through refolding of denatured PrSc as well as modifying the folding of PrPc, recombinant PrP90-228 and synthetic PrP peptides. We hope to establish the size and composition of the smallest infectious prion particle using Zn2+ or diethylpyrocarbonate (DEPC) to perturb reversibly the structure of PrPSc. The results with Zn2+ ions and DEPC as well as those obtained with synthetic PrP peptides implicate His111 in the synthesis of PrPSc and the propagation of scrapie prion infectivity. Radiolabeled DEPC will be used to test the hypothesis that scrapie infectivity is diminished when His111 is carbethoxylated. Studies of the molecular mechanisms responsible for prion diseases should give new insights into the etiology, pathogenesis and treatment of the more prevalent neurodegenerative disorders such as Alzheimer's disease.